1. Field of the Invention
This invention relates to phenolic foams and is more particularly concerned with a novel process for producing non-punking phenolic foams.
2. Description of the Prior Art
Phenolic foams, and their various modes of preparation beginning with a resole or a novolac resin, are well known in the art; see, for example, Plastic Foams Part II, pp 639 - 673, edited by K. C. Frisch and J. H. Saunders, 1973, Marcel Dekker Inc., New York, N. Y. Phenolic foams are most commonly produced by the acid curing of a resole resin, and, generally speaking, strong mineral acids have been found to be the most efficient catalysts. But, at the same time, the foams tend to overheat which causes their distortion, cracking, and splitting, especially when formed in anything but small amounts or thin sections so as to allow heat dissipation.
U.S. Pat. No. 2,772,246 discloses the use of combinations of various organic phosphorus acid esters with a mineral acid or phosphoric acid as a means of controlling foam cell size and uniformity during the foam formation. U.S. Pat. No. 3,298,973 teaches the use of boric acid in combination with organic hydroxy acids as resole curatives and also as a means to reduce punking. U.S. Pat. No. 2,933,461 discloses mixtures of phenol sulfonic acid with benzene sulfonic acid or sulfuric acid as useful catalysts. Some art teaches the removal of vaporizable acidic catalysts from the formed phenolic foams by treatment of the foams at high temperatures; see U.S. Pat. No. 3,522,196. U.S. Pat. No. 3,673,130 discloses the use of various phosphorus compounds in conjunction with known acid catalysts to provide non-punking foams. The term "punking" refers to the phenomenon observed when a piece of foam which has been removed from a flame continues to glow and combust without a visible flame. This phenomenon will be discussed in greater detail hereinbelow.
While the prior art discloses the preparation of phenolic foams which have good fire resistance properties including non-punking characteristics and which can be produced within desired density ranges with fine uniform cells, as typically noted in U.S. Pat. No. 3,673,130, such foams are severely limited in the batch size in which they can be prepared. When it is desired to scale-up their preparation above the bench scale level, that is to say preparations greater than a few pounds of foam contained in a thick section, the foams split and crack regardless of what heating cycles or curing steps are employed. This is caused by the high and rapid exotherm arising from the curing reaction of the resole resin, which curing reaction is brought to its peak very rapidly by the prior art catalysts.
U.S. Pat. No. 4,001,148 discloses acid curing agents that call for mixtures of a strong inorganic or organic acid, a glycol, and phosphoric acid wherein the novelty resides in the inclusion of the glycol in order to avoid cracking and bursting of the formed foam block.
It has now been discovered that non-punking phenolic foams of fine uniform cell and controllable densities can be prepared in large mass without cracking or splitting. Further, in contrast to the prior art, it has been discovered that the foams can be prepared using reactants at room temperature. Any heat curing of the resulting foam is purely optional and not necessary to achieve a full thermoset condition.
In a further distinguishing feature over prior art phenolic foams, those of the present invention are characterized as being essentially non-corrosive in terms of the materials with which they come into contact.
The advantageous properties of the present foams are achieved without the need for employing both the corrosive phosphoric acid and a glycol. Although the use of a glycol is not precluded in the present invention.